A street in Calcutta floods during monsoon season. After some decades of increasing rainfall, climate change could bring drier monsoons, said Jacob Schewe from the Potsdam Institute for Climate Impact Research. Credit: Mark E Dyer/Flickr

Monsoon rains in India may fail more frequently as climate change proceeds into the 22nd century, German researchers said this week. That danger could be critical for farming in what is set to become the world’s most highly populated country by 2030, and would follow an already expected wetter period. “Previous studies showed that Indian monsoon rainfall would increase more or less linearly with global warming over the next century,” said Jacob Schewe from the Potsdam Institute for Climate Impact Research. “The monsoon can respond to climate change in a more complicated way. We’ve seen that it matters to look further into the future.”

In South Asia, summer monsoon rains fall as winds blow from the southwest Indian Ocean over the continent between June and September. They end when the wind direction reverses in September or October. What Indian monsoon rain seasons will do as the world warms is an important and difficult question that many researchers are trying to answer. Though more rainfall has been predicted, recent years haven’t matched that expectation. While factors like pollution have an effect, changes climate scientists already know a major climate pattern plays a very important part in monsoons.

“There is a coupling between the El Niño Southern Oscillation and the monsoon that’s been observed for a long time,” Jacob told me. In years when El Niño occurs, an air movement pattern called the Walker circulation pattern gets shifted eastward. That brings high pressure over India and weakens the monsoon. While some changes in El Niño are already happening, the Walker circulation is expected to weaken, but not for some time yet. That could mean scientists’ climate models don’t pick up its effects. “People have looked at monsoon changes but not many studies have looked beyond 2100,” Jacob said. “You really have to consider longer timescales – beyond 2100 – to assess the full range of consequences for the monsoon.” Read the rest of this entry »